plot specific frequency of signal

I would like to plot the 0 to 50 Hz of ecg signal by FFT. How Can I change it that plot the 0 to 50 Hz frequencies.

this is my code

a1 =importdata ('ecg.txt');

Fs = 500; % Sampling Frequency

Fn = Fs/2; % Nyquist Frequency

L = numel(a1);

nfft = 2^nextpow2(L);

y=fft(a1,nfft)/L;

y1=abs(y);

f=(0:nfft/2)*Fn*2/nfft;

figure

plot (f,y1(1:numel(f))

title ('‘Frequency Domain’')

xlabel('f(Hz)')

ylabel('|y1(f)|')

grid

Answer 1

[Star Strider]

Are you certain that 500 Hz is the correct sampling frequency for this trace?

That would calculate to a heart rate of 335 bpm!

That aside, one way to analyse it —

a1 = readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1317805/ecg.txt');
% EKG file found in: I would like to calculate heart rate by determining threshold for amplitude
% https://www.mathworks.com/matlabcentral/answers/1925115-i-would-like-to-calculate-heart-rate-by-determining-threshold-for-amplitude?s_tid=srchtitle

Fs = 500;                                           % Sampling Frequency
Fn = Fs/2;                                          % Nyquist Frequency
L = numel(a1);
Duration_sec = L/Fs

Duration_sec = 12

R = islocalmax(a1, 'MinProminence',0.75);
HR = nnz(R)/Duration_sec * 60                       % BPM

HR = 335

nfft = 2^nextpow2(L);                               
y=fft((a1-mean(a1)).*hann(L),nfft)/L; 
y1=abs(y); 
f=(0:nfft/2)*Fn*2/nfft;
figure
plot (f,y1(1:numel(f))) 
title ('‘Frequency Domain’')  
xlabel('f(Hz)') 
ylabel('|y1(f)|') 
grid

t = linspace(0, L-1, L)/Fs;                     % Time Vector

figure
plot(t, a1)
hold on
plot(t(R), a1(R), '^r')
hold off
grid
xlabel('Time (s)')
ylabel('Amplitude (mV)')

figure
plot(t, a1)
grid
xlabel('Time (s)')
ylabel('Amplitude (mV)')
xlim([0 1])

.